Communications systems often utilize a technique in which a relatively large central component such as a switch provides operating power to multiple smaller, distributed terminal devices such as telephones. The power is typically provided in direct-current (DC) form over the same conductors that carry the communications signals. It is often considerably more efficient, both operationally and financially, to use a central power supply and distribution circuitry rather than using independent power supplies in each terminal device. Such remote powering has been used in traditional analog telephone systems, and is used today in Internet Protocol (IP) telephone systems for example.
Modem communications systems that employ remote device powering also employ circuits and methods that enable the switch or other central component to discover whether a terminal device is attached to a particular interface before providing power to that interface. Safety and operational efficiency are improved when power is not applied to unconnected/unused interfaces. Generally, discovery involves sensing some aspect of the interface that necessarily has one of two distinct states depending on whether a remotely powered device is present. This can be as simple, for example, as sensing a logic level, or it may involve a more complex interaction of software or firmware processes.
Ethernet technology is widely used as a physical-layer communications medium in IP telephone systems. At present, standard Ethernet interfaces employ unshielded twisted pair (UTP) cables in which multiple conductor pairs are utilized to carry the data signals, as well as remotely supplied power when present. Example of such physical layer interfaces include so-called 10Base T and 100Base T interfaces in which two or more pairs of conductors carry data and power signals from the switch to the telephone, and from the telephone to the switch. From the perspective of either device, one of these pairs may be denoted the “transmit” pair and the other the “receive” pair, depending on the direction that data travels with respect to the device. For the 1000Base T physical layer interface, the transmit/receive distinction does not apply, because data flows in both directions at the same time, while power flows in one direction at all speeds. The physical cable is itself categorized in a standardized fashion using the well-known “Category N” terminology, where N is in the range of 3 to 6, with Category 6 cable being the most modem and capable of carrying very high data rate signals with good fidelity.
In the UTP Ethernet interfaces of IP telephone systems, it has been known to use two specific types of discovery circuitry, referred to as “common mode” and “differential mode” discovery circuitry. Both types utilize both the transmit and receive conductor pairs of an interface (or two pairs out of the 4 pairs to detect the presence of a device in need of inline power). In common mode discovery, the switch includes a signal generator connected between respective center taps of isolating transformers for the two pairs that are located in the switch, and the telephone includes a resistor similarly connected between respective center taps of isolating transformers located in the telephone for the two pairs of conductors. In operation, the switch applies a voltage V1 followed by a voltage V2 across the conductor pairs while measuring the respective currents I1 and I2, and uses the voltage and current differences to calculate the value of the attached resistance (i.e., R=(V2−V1)/(I2−I1)). In differential mode discovery, the switch includes a pulse generator connected to one of the conductor pairs of the interface, and the telephone includes loopback switches that create a connection between the two conductor pairs. A discovery pulse generated by the pulse generator of the switch on the one conductor pair is returned to the switch on the other conductor pair, where its presence or absence can be detected as an indication of the presence or absence of the telephone.